Proportional throttle valves are often used when the level of the fluid flow passing through these valves is to be continuously changed, depending on an existing magnetic current of a coil activating the valve piston. The opening cross section of the valve seat of the valve piston is dependent on the position of the valve piston. If the coil is supplied with a direct electrical current, a magnetic force proportional to the magnetic current forms and moves the valve piston into an open position. In general a spring located on the back of the valve piston is tensioned generating a spring force counteracting the magnetic force. If an equilibrium is established between the magnetic force and spring force, the valve piston remains in its position.
The proportional throttle valve is largely independent of the pressure level to be controlled because the valve piston is pressure-equalized, i.e., a uniformly high pressure acts on the two end surfaces of the valve piston.
Proportional throttle valves are used, for example, in lifting-lowering applications, such as in industrial trucks. To lower the load for these applications, a 2/2-way directional valve is used for the basic lifting-lowering motion. At the same time, the proportional throttle valve is used for controlling the volumetric flow.
In proportional throttle valves, the problem is that the volumetric flow is dependent on the differential pressure between the fluid inlet and fluid outlet. For example, in industrial trucks or other lifting devices, the lowering speed depends on the load being moved. Boundary values could damage the lifting device or the load to be transported in the lowering process and can be exceeded. In the prior art, a non-adjustable maximum volumetric flow regulator is connected in series upstream of a proportional throttle valve. The maximum volumetric flow regulator is designed to be passive and engages only when the maximum lowering rate is exceeded. A hydraulic circuit set up in this respect according to the prior art is therefore complex and, due to the plurality of components, requires an installation space of corresponding large dimensions.